This invention relates to packaging material for heating or cooking of food by microwave energy. It is particularly directed to microwave active film or wrapping materials which provide a level of heating which can be varied to match the heating requirements of a variety of foods.
A wide range of prepackaged refrigerated or frozen foods has long been commercially available. Such foods may be heated in conventional gas or electric ovens, or more recently in microwave ovens. However, suitable packaging of multi-component meals for microwave cooking has been an elusive goal. Different foods respond to microwave energy in different ways, depending on their physical and electrical properties, mass, shape, and other parameters. Different foods also require different amounts of heating in order to reach a suitable, customary serving temperature. For example a fruit dish may require defrosting but little or no heating above room temperature. A meat entree should be heated to about 100.degree. C. Vegetables should likewise be heated to near 100.degree. C., but care should be taken that they do not become overcooked or dry. Bread products should have a hot, crisp crust and an interior that is not overheated or dried out.
There has been a long-felt need for a practical microwave packaging material that can be readily adapted to the heating and cooking requirements of a variety of diverse foods. Many attempts have been made to achieve this result by indirect means, such as by providing shielding of food components or by selective spacing of foods within a package. For example, U.S. Pat. No. 3,219,460, Brown, teaches heating of two or more frozen food items using a multi-compartment electrically conductive tray, each compartment being shielded with a top made of an electrically conductive material with several openings to regulate access to high frequency waves.
U.S. Pat. No. 3,271,169, Baker, discloses varying food spacing from an underlying conductive layer or ground plane. Dielectric spacers may be employed, the food products may be located on various heights above a conductive sheet, or the conductive sheet may be at different distances below the different foodstuffs.
U.S. Pat. No. 3,302,632, Fichtner, discloses the uniform cooking of different foods by providing a cooking utensil the walls of which regulate microwave transmission to the food. High conductivity grids of different mesh are used to dampen the microwaves.
U.S. Pat. No. 4,190,757, Turpin, discloses a package which includes a metal foil shield having holes of a selected size to provide a predetermined controlled amount of direct microwave energy to the food.
U.S. Pat. No. 4,656,325, Keefer, discloses a pan with a cover which is said not to transmit refected microwave energy. The cover can be comprised of a dielectric substrate having metal powder or flakes dispersed therein and can bear an array of conductors comprising a plurality of spaced-apart, electrically conductive islands.
U.S. Pat. No. 3,547,661 Stevenson, discloses a container for heating different items to different temperatures simultaneously comprising a cover of a radiation reflecting material having apertures in opposite walls formed in the material. Food items are selectively placed in or out of alignment with the apertures.
European Patent Application 206 811, Keefer, discloses a container for heating material in a microwave oven, comprising a metal foil tray with two rectangular apertures. The container lid is a microwave transparent material having two metallic plates located thereon, in registry with the apertures.
Various types of films or sheets have been disclosed which are useful as lids or wraps for microwave cooking. For example, U.S. Pat. No. 4,518,651, Wolfe, discloses a flexible composite material which exhibits a controlled absorption of microwave energy based on presence of particulate carbon in a polymeric matrix bound to a porous substrate. The coating is pressed into the porous substrate using specified temperatures, pressures, and times, resulting in improved heating.
U.S. Pat. No. 4,735,513, Watkins, discloses a flexible sheet structure comprising a base sheet having a microwave coupling layer and a fibrous backing sheet such as paper bonded thereto to provide dimensional stability and prevent warping, shriveling, melting or other damage during microwave heating.
European application 0 242 952 discloses a composite material for controlled generation of heat by absorption of microwave energy. A dielectric substrate, e.g., PET film, is coated with a metal in flake form, in a thermoplastic dielectric matrix. The use of circular flakes with flat surfaces and smooth edges is preferred. Flakes of aluminum are disclosed.
U.S. Pat. No. 4,267,420, Brastad, discloses a plastic film or other dielectric substrate having a very thin coating thereon which controls the microwave conductivity when a package wrapped with such film is placed within a microwave oven.